Vitamin A, which is a fat-soluble vitamin, is easily soluble in organic solvent and fat, and is insoluble in water. Vitamin A is perishable when exposed to light, heat, acid, or oxidant, so it is generally required to be prepared into a microcapsule form to use it.
The microcapsulation of vitamin A is generally, first heating vitamin A crystals, accessory oil and aqueous solution containing protective colloid together, emulsifying, and then spray drying the emulsion, to obtain the microcapsule.
China patent CN1965657(A) describes a method for preparing vitamin A microcapsules, said method is adding vitamin A oil into a solution of modified starch pre-formulated several hours earlier, and dispersing and emulsifying at high speed with a rotating speed of 5000˜20000 rpm, then homogenizing twice at 10˜40 MPa at room temperature, and finally centrifugal spray drying, to obtain the vitamin A microcapsule. Due to the fine particle size of the obtained product, it is mainly used for the fortification of flour.
China patent CN101214219(A) reports a method for preparing vitamin A, vitamin E microcapsules, and the emulsifying process thereof employs 10000˜20500 rpm high-speed shearing, and homogenization at 40˜60 MPa for 3 times is required, and then the microcapsule is prepared by spray-drying.
The above-mentioned method for preparing vitamin A microcapsules by high-speed shearing emulsification plus high-pressure homogenization and then spray-drying, has the following insurmountable problems:
1) the emulsifying process is carried out batchwise in an open environment, and the emulsifying time for a single batch is long, and when emulsifying, the temperature at the shearing site is high, which causes vitamin A perishable;
2) the motor power required by the high speed shearing machine and the high pressure homogenizer is high, and the energy consumption is high;
3) due to the batchwise operation, during the spray-drying process after completing the emulsification, the emulsion is prone to stratify, and the gathered small oil bead is easily coalesced into large particles, thus affecting the embedding effect and the bioavailability of the final product.
4) It is difficult for vitamin A to disperse into the microcapsule in nano-scale dimensions by the high pressure homogenization, thus affecting the use thereof in certain products.
In view to the above-mentioned problems, the present inventors have proposed a method for continuously preparing nano-dispersed vitamin A microcapsule in CN101513394. In this method, vitamin A crystals are first ground with antioxidant and solvent into a dispersion liquid of vitamin A, then the above-mentioned dispersion liquid is cooled by a pump after pre-heating and warming to dissolve, then pumped into a high gravity rotary bed crystallization device, meanwhile an aqueous solution containing protective colloid is pumped into the same high gravity rotary bed crystallization device, to obtain a nano-dispersed dispersion liquid of vitamin A at the outlet. The dispersion liquid is spray-dried in a spray dryer equipped with a fluidized cooling device, to obtain the nano-dispersed microcapsule of vitamin A. This invention employs a high gravity rotary bed crystallization device as the means for the nano-crystallization of vitamin A, so that the bioavailability of vitamin A is increased, and the applicability of the product is broadened.
However, vitamin A is a fat-soluble vitamin with poor stability, even after the microencapsulation by spray-drying, the effective content thereof will still significantly decrease during the storage, due to the effects of light, heat, oxidant, and acid. Through further studies, we found that, if avoiding the long-time heating and minimizing the chances to contact with oxygen during the microcapsulation of vitamin A, meanwhile increasing the oxygen-preventing and water-preventing measures of the outer layer of microcapsule, the stability of vitamin A can be effectively increased.